Research will be performed on the regulation of animal cell proliferation. We have evidence that animal cells in tissue culture are committed to divide or not divide at a point (R) in the G1 part of their cell cycle. Evidence will be sought regarding the conditions that determine this event and the timing of the event in the cycle. Experiments on the subcellular basis for the regulation will seek alterations of proteins in the membrane and in chromatin, in synchronously growing cultures. We intend also to study the changes in metabolic pools and of cytoplasmic enzymes that might be related to DNA initiation. Use will be made of mutant cell lines that we intend to isolate, and also of proliferation-stimulating and inhibiting factors. The effects of chemical mutagens, of carcinogens, and of agents that preferentially kill tumor cells in vivo will be utilized to examine the nature of this regulatory event and its possible malfunction in malignant cells. We expect cancer cells to be arrested at other places in the cycle, and as a consequence to be more sensitive to agents that are toxic to cells not arrested at the normal stopping point. Studies of the membrane in relation to normal and unrestrained growth will be continued, both to give evidence for membrane alterations related to growth, and to test the hypothesis that growth control might depend to a great extent on entry of regulatory substances.